ARM: at91/DT: atmel_usba correct atmel,vbus-gpio meaning

atmel,at91sam9rl-udc is a USB gadget, it has no means to control vbus.
atmel,vbus-gpio is in fact used to detect the presence of vbus.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
[nicolas.ferre@atmel.com: modify the commit message]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>